(1) Field of the Invention
The present invention relates to the field of rotary wing aircraft having at least one rotor driven in rotation by a power plant. The present invention relates more particularly to servo-controls fitted to rotorcraft rotors in order to vary the angle of incidence of the blades of such a rotor.
(2) Description of Related Art
Rotorcraft are fitted with at least one main rotor having an axis that is substantially vertical and that provides the rotorcraft at least with lift. In the specific circumstance of a helicopter, the main rotor also provides the rotorcraft with propulsion and with guidance in pitching and in roll.
Rotorcraft are also commonly fitted with at least one anti-torque device as formed by an auxiliary rotor with an axis that is substantially horizontal and suitable also for providing the rotorcraft with guidance in yaw. By way of example, such an auxiliary rotor may be a tail rotor, and also by way of example it may indeed be a propulsive propeller in a helicopter having high speed propulsion.
In order to vary the attitude in flight of the rotorcraft, the pilot operates flight controls that cause the angle of incidence of the blades making up the rotary wing of said at least one rotor to vary. The pilot may be a human pilot or an autopilot. The angle of incidence of the blades is varied by feathering (moving) the blades, i.e. by causing their pitch to vary either cyclically or collectively. More particularly, the blades are hinge-mounted on the rotor so as to be capable of pivoting about their own longitudinal axes. The pivoting of the blades about their own axes in association with their rotation about the axis of rotation of the rotor gives rise to variation in their pitch that may be cyclic and/or collective.
For this purpose, and considering by way of example a main rotor, a swashplate comprises a non-rotating bottom plate associated with a rotating top plate serving to modify the angle of incidence of each of the blades at as a function of its individual position in azimuth. The bottom plate is controllable along three distinct flight control lines and the top plate is connected to each of the rotor blades by respective linkages. The bottom plate may be moved in any orientation in three dimensions relative to the mast of the rotor on which the bottom plate is guided.
Such an arrangement of the swashplate gives it freedom to move axially while also being capable of moving in all directions like a ball joint so as to vary the pitch of the blades depending on the flight commands issued by the pilot. The forces for moving the swashplate are potentially large, so it is useful to assist the pilot in delivering the force needed to feather the blades.
That is why it is common practice to use servo-controls for varying blade pitch. Servo-controls can assist a human pilot in terms of the forces that the pilot needs to deliver in order to vary blade pitch via a mechanical force transmission linkage. The servo-controls may also be operated by an actuator as a function of flight commands issued by an autopilot.
Conventionally, such a servo-control comprises a hydraulic actuator having at least one double-acting actuator cylinder fed with fluid under pressure from a fluid source of the rotorcraft. More particularly, the fluid is an incompressible fluid, specifically oil, enhancing the power and the accuracy obtained for feathering the blades.
The hydraulic actuator is potentially an actuator having a single actuator cylinder or an actuator having multiple actuator cylinders, each actuator cylinder housing a rod. Relative movement between the rod and the actuator cylinder receiving it is obtained by admitting fluid into the inside of the actuator cylinder, said relative movement being used to convert the hydraulic power developed by the servo-control into mechanical power for feathering the blades.
In a servo-control comprising a hydraulic actuator having multiple actuator cylinders, the actuator cylinders may for example be arranged in tandem, being combined in a common actuator cylinder and housing a common rod. Also by way of example, the actuator cylinders of a hydraulic cylinder having multiple actuator cylinders may be arranged in parallel, each receiving a respective rod, with the rods being mechanically connected together.
The actuator cylinder(s) is/are double-acting, being individually fed with fluid by a corresponding hydraulic circuit conveying fluid under pressure in both directions between the actuator cylinder and the fluid source. For return of the fluid to the fluid source, it is possible for the circuit to be at atmospheric pressure.
The rod is arranged as a piston and is mounted to move axially in both directions inside an actuator cylinder that receives it. The actuator cylinder has fluid flow passages at its opposite ends referred to as its top and its bottom. Relative movement between the rod and the actuator cylinder that receives it is caused to take place along each of the directions in which the rod can move by admitting fluid into the actuator cylinder via its top passage or via its bottom passage.
Depending on the ways in which the servo-control is installed on board the aircraft, the rod may be fastened to a structure of the rotorcraft in order to feather the blades by moving the actuator cylinder. Conversely, in a variant, the actuator cylinder may be fastened to the structure of the rotorcraft in order to feather the blades by moving the rod.
The operation of the hydraulic actuator depends on one or more hydraulic valves (valve distributor) included in the servo-control. Each actuator cylinder is fed individually with fluid under pressure by a hydraulic valve associated therewith. Such hydraulic valves may in particular be of the slide type or they may be of the rotary type, and they serve to direct the fluid delivered under pressure through the hydraulic circuits, thereby varying the hydraulic power developed by the hydraulic actuator.
Each hydraulic valve is placed on the hydraulic circuit between the fluid source and an actuator cylinder associated therewith, so that the supply of fluid under pressure to an actuator cylinder is governed by a hydraulic valve that is specific thereto.
Each hydraulic valve receives the fluid under pressure from the hydraulic circuit in which it is placed in order to feed fluid to the associated actuator cylinder, the fluid passing selectively through one or another of the passages. Conversely, each hydraulic valve discharges the fluid it receives from the actuator cylinder that is associated therewith, returning it to the fluid source.
By way of example, reference may be made to the following documents: U.S. Pat. No. 3,529,514 (Mayo Millard G. et al.), U.S. Pat. No. 4,128,047 (Caero J. G.), and FR 2 444 825 (Messerschmitt Boelkow Blohm), which describe such hydraulic valves as commonly used in the field of aviation.
More particularly, for a given hydraulic valve, a valve cylinder of the hydraulic valve has first ducts for putting the hydraulic valve into fluid flow communication with the fluid source. An admission duct is dedicated to admitting into the hydraulic valve fluid under pressure coming from the fluid source, while a discharge duct is dedicated to discharging fluid from the actuator cylinder out from the hydraulic valve and back to the fluid source.
The hydraulic cylinder also has second ducts for putting the hydraulic cylinder into fluid flow communication with the actuator cylinder via passages provided therein. One second duct is dedicated to establishing fluid flow communication between the hydraulic valve and the top passage, while another second duct is dedicated to establishing fluid flow communication between the hydraulic valve and the bottom passage. Such a hydraulic valve is typically a hydraulic valve having four ports and N stable positions, where N is not less than two.
The valve cylinder also movably receives at least one main valve member that is movable by a control member. When the servo-control has a plurality of hydraulic valves allocated to respective actuator cylinders, the control member engages each of the main valve members of the hydraulic valves jointly, and is itself movable by the pilot using flight controls to move each of the main valve members jointly.
The control member, possibly arranged as a connecting rod, may for example be movable by said force transmission linkage when using manual flight controls. Also by way of example, the control member may be moved by a said actuator, such as an electric motor, when using automatic flight controls.
The main valve member provides first fluid flow channels through the hydraulic valve between the first ducts and the second ducts.
Depending on the position of the main valve member as controlled by the control member, the admission duct and the discharge duct are put into fluid flow communication selectively with one or the other of the second ducts.
As a result, moving the main valve member by means of the control member complying with the flight controls operated by the pilot serves to direct or to vary the flow fluid through the actuator cylinder associated with the hydraulic valve. Thus, the hydraulic valve makes it possible to adjust the relative movement between the actuator cylinder and the rod it receives in one or the other of the directions in which the rod can move inside the actuator cylinder.
In this context, consideration is generally given to a normal situation in which the servo-control is operating normally without any failure in the or one of the hydraulic valves included in the servo-control. However, such a failure can be observed in particular in the event of the main valve member seizing inside the valve cylinder of one of the hydraulic valves.
For a servo-control having a hydraulic actuator with a single actuator cylinder, a malfunction of the servo-control or of the fluid feed circuit prevents the blades being feathered by means of the servo-control. The pilot can nevertheless feather the blades by forcing the rod to move by means of the force transmission linkage. Nevertheless, it should be understood that such forced movement of the rod is uncomfortable for the pilot.
Incorporating a plurality of actuator cylinders in the hydraulic actuator makes it possible to make the operation of the servo-control safer, in particular in the event of a malfunction of one of the hydraulic circuits feeding fluid to one or the other of the actuator cylinders, or indeed in the event of a malfunction of one of the actuator cylinders.
There thus arises the problem of making safe, reliable, and accurate the operation of the servo-control. More particularly, in the event of the main valve member of the or one of the hydraulic valves seizing, that must not cause the hydraulic actuator to be blocked, nor must it cause it to malfunction by allowing uncontrolled movement of the rod(s) co-operating with the respective actuator cylinder(s).
That is why, in order to make the operation of servo-controls more reliable, it is known to provide double hydraulic valves. A double hydraulic valve includes an emergency valve member mounted inside the valve cylinder, being interposed between the main valve member and the valve cylinder. The emergency valve member has second channels communicating respectively with the first channels of the main valve member.
More particularly, the main cylinder member is guided to move inside the emergency valve member, which is itself potentially movable inside the valve cylinder. In the nominal position of the hydraulic valve without any seizing of the main valve member, the emergency valve member is held in a predefined position inside the valve cylinder by positioning means. Such positioning means may in particular be of the elastically deformable type that bear in opposite directions against the valve cylinder and against the emergency valve member.
In the event of the main valve member seizing inside the emergency valve member, the emergency valve member is entrained by friction by the main valve member against forces exerted by the elastically deformable means against the emergency valve member.
Such arrangements make it possible to keep the hydraulic valve in operation in spite of the main valve member seizing.
Nevertheless, in the event of the main valve member seizing, the operation of the hydraulic valve that is obtained complies only partially with the flight commands issued by the pilot.
It can be seen that the extra reliability in operation of the hydraulic valve could still be improved further. The above-mentioned arrangement of the hydraulic valve serves essentially to avoid the hydraulic valve becoming blocked in the event of the main valve member seizing in the or one of the hydraulic valves included in the servo-control.
With a hydraulic actuator having multiple actuator cylinders, proper control of the operation of the hydraulic actuator can be achieved by one of the hydraulic valves in the event of the main valve member in the other hydraulic valve seizing. It is possible that proper control could be obtained by the failed hydraulic valve in the event of the main valve member, when seized, is in an appropriate position inside the emergency valve member; however its position is randomly uncertain.
As a result, in order to obtain good operation of the hydraulic actuator in the event of one of the hydraulic valves malfunctioning, it is desirable for seizing of the main valve member in one of the hydraulic valves never to constitute an obstacle that prevents proper operation of the other hydraulic valve.
Furthermore, the use of positioning means of the elastically deformable type for holding the emergency valve member inside the valve cylinder in said predefined position needs to be improved.
In the event of the main valve member seizing, it is desirable for the hydraulic valve to be controllable using forces that are small, so as to improve the comfort of a human pilot operating flight controls manually and so as to avoid overdimensioning said actuator when using automatic flight controls.
For a description of a technological environment close to the present invention, reference may be made to Document FR 2 916 492 (Eurocopter France), which describes a device for detecting seizing of a double hydraulic valve in a servo-control for a hydraulic actuator having two double-acting actuator cylinders for feathering the blades of a rotorcraft rotor.